In recent years, the demand for material that facilitates bright full-color display with excellent color purity and with low power consumption has been increasing. For example, light-emission type elements such as CRTs, LCDs, PDPs and ELDs have such characteristics that they are bright and thereby easy to see, and therefore a number of technologies have been proposed in the past. However, above-mentioned various light-emission type elements have a problem that since the light emission needs to be directly looked at, they causes visual fatigue when viewed for many hours. Further, since mobile devices such as mobile phones are often used outdoors, there is another problem that the light emission is balanced out under sunlight and the viewability thereby deteriorates. Meanwhile, among the light-emission type elements, LCDs are the technology that has been especially growing in demand, and are used for various display applications including large and small displays. However, LCDs have a problem that the viewing angle is narrow, and thus they have problems in terms of viewability that need to be improved in comparison to other light-emission type elements.
Meanwhile, although the role of paper used for storing and conveying documents has been decreasing because of the widespread use of computers in offices, the tendency to print and read digital information in paper is still persistent when such information is perused. Therefore, the amount of paper that is temporally used and abandoned immediately after the use shows, on the contrary, an upward trend in recent years. Further, the amount of paper that is consumed daily for books/magazines/newspapers and the likes is seen as a threat in terms of natural-resources/environment, and they do not seem to decrease unless the medium is changed. However, when the way of information recognition and the way of thinking by the human being are taken into consideration, the superiority of “paper” over “display” typified by CRTs (Cathode Ray Tubes: Braun tubes) and transmission type liquid crystal displays cannot be ignored.
Therefore, the electronic paper in which the merit of paper and the merit of displays, which can directly handle digital information, are combined is expected to be put into practical use as an alternative to the paper in recent years. The necessary characteristics expected in the electronic paper include being a reflection type display element, having high reflectivity to while light and high contrast ratio, being capable of displaying with high definition, having memory effect in display, being capable of driving with a low voltage, being thin and light, being inexpensive, and so on.
The display methods of electronic paper include a reflection type liquid crystal method, an electrophoresis method, a two-color ball method, an electrochromic method, and so on. The reflection type liquid crystal method includes a G-H type liquid crystal method using dichromatic pigment, a cholesteric liquid crystal method, and so on. These methods have an advantage that they do not need to use a backlight and thus consume smaller electrical power in comparison to the light-emission type liquid crystal method. However, these methods involve the dependence on viewing angle and have low light reflectivity, and thereby have a problem that the screen inevitably becomes darker. The electrophoresis method exploits a phenomenon called “electrophoresis” in which white pigments, black toner, or the like is moved on electrodes by the effect of an electric field. The two-color ball method includes a spherical body painted with two colors such as white and black in a half-and-half fashion, and uses the rotation by the effect of an electric field. Both the methods have a merit that they consume low electrical power and that they do not involves the dependence on viewing angle. However, in these methods, it is necessary to arrange enough gaps to let particulate bodies get therein, so that they cannot be closely packed and thus making achieving a high contrast very difficult. Further, when it is to be displayed in full color, a juxtaposition mixture method using a color filter is adopted, and thus posing a problem that the reflectivity decreases and the screen inevitably becomes darker.
The electrochromic method exploits reversible oxidation-reduction reactions caused by an application of an electric field, and thereby their accompanying color development/disappearance. Further, electrochromic (hereinafter, simply called “EC”) elements have been used in dimming mirrors of automobiles, clocks, and the likes in the past. The display by such an EC element does not require a polarizing plate and the like, does not involve the dependence on viewing angle, is a light reception type and thus superior in terms of viewability, has a simple structure, and is easily constructed in a large size. Further, it has another merit that light emission of various color tones is possible by selecting proper material.
To display in full color in an EC element, pigments capable of developing colors including cyan (C), magenta (M), and yellow (Y), which are used in subtractive color mixture, are applied. Then, by stacking C, M and Y color-development layers, a display device that can develop colors in full color is obtained by a lamination mixture method. Alternatively, by putting each of the coatings of C, M and Y color-development layers in different areas, a display device that can develop colors in full color is obtained by a juxtaposition mixture method. For example, black can be displayed by mixing colors of C, M and Y. Further, white can be displayed by bringing each pigment to a color disappearance state so that the background color becomes white. Since the EC element is a reflection type display element in which color development/disappearance is electrically repeated as described above, they are advantageous in terms of burden put on eyes and in terms of contrast.
Further, the research on material called “π-electron based conjugated polymer” as material for color-development layers has been in progress in recent years. Among the materials, ones that exhibit an electrochromic characteristic have been known, and EC elements for which a film is produced from those materials and used as a color-development layer have been also known. To display in full color by an EC element using those materials, it is necessary to adopt a structure in which C, M and Y color-development layers are stacked and to use the lamination mixture method to display in full color as described above. Alternatively, it is necessary to put each of the coatings of C, M and Y color-development layers in different areas and to use the juxtaposition mixture method to display in full color.
However, in accordance with the lamination mixture method, the overall structure of the EC element becomes complicated. Further, since the laminated body is used, loss of the light is inevitably increased due to absorption and scattering by portions other than the EC layer such as an electrode portion and a substrate portion, and thus imposing a limit to the increase in the efficiency of use of light as reflective type display elements. On the other hand, in accordance with the juxtaposition mixture method, unintended loss of the light is not caused, though it is necessary to provide division walls and to put each of the coatings of C, M and Y color-development layers in different areas. Therefore, it is possible to provide reflection type display devices capable of displaying in full color for which the efficiency of use of light is improved.
In the past, when the juxtaposition mixture method is applied, the following manufacturing method, for example, is used as a publicly-known technology. Firstly, a substrate on which a conductive layer is formed in advance is prepared, and the conductive layer is etched and thereby patterned into a desired shape. Then, division walls are provided on it by a technique such as photolithography. After that, each of the coatings of C, M and Y color-development layers is applied in different areas to manufacture an EC element. Further, in a case where a π-electron based conjugated polymer is used as the C, M and Y color-development layers, a method of forming a film by applying and drying a liquid in which a π-electron based conjugated polymer is dissolved or dispersed to a high degree, a method of forming a film by electrochemically polymerizing a π-electron based conjugated monomer, or a similar method is typically used as a conventional method of forming a film of a π-electron based conjugated polymer (see Patent Citations 1, 2, 3 and 4).    [Patent Document 1]    Japanese Unexamined Patent Application Publication No. 2002-287173    [Patent Document 2]    Japanese Unexamined Patent Application Publication No. 2006-208862    [Patent Document 3]    Japanese Unexamined Patent Application Publication No. 2006-058617    [Patent Document 4]    Published Japanese Translation of PCT International Publication for Patent Application, No. 2004-527902